A truly unique case--yes or no?

[robocopkorea]

This car has 265 horsepower.
Why isn't horsepower countable and thus take an "s"?
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100 horses 5 powers of Superman
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The meaning is like the "uncountable power", but then you shouldn't be able to put a number in front of it.
He has a lot of power in his arms.
This plant produces a lot of (electrical) power.
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This office ( job-position ) has 3 powers. One is the power to hire someone. The second is the power to fire someone. However, the most interesting one is the power to declare a one day holiday for the workers twice a year.
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By all rights, horsepower should have an "s". Either that, or you should not be able to put a number in front of it!
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[Ouisch]

Some texts that I have read indicate that many years ago "horse" was considered a mass noun, like "cattle" and "water."

[DavyBCN]

This car has 265 horsepower.
Why isn't horsepower countable and thus take an "s"?
----------------------------------------------------------------------
100 horses 5 powers of Superman
----------------------------------------------------------------------
The meaning is like the "uncountable power", but then you shouldn't be able to put a number in front of it.
He has a lot of power in his arms.
This plant produces a lot of (electrical) power.
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This office ( job-position ) has 3 powers. One is the power to hire someone. The second is the power to fire someone. However, the most interesting one is the power to declare a one day holiday for the workers twice a year.
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By all rights, horsepower should have an "s". Either that, or you should not be able to put a number in front of it!
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What the sentence really means is that the car has 265 units of horsepower. Unit is countable so can be plural. Horsepower is uncountable. Why? Because it is! My students here in Spain have terrible problems with the word travel - which is an uncountable noun and verb in English but a countable noun and verb in Spanish. When they ask me why - I say "because it is!"